Modeling the ionospheric delayed response to solar rotation period changes in EUV radiation

The thermosphere-ionosphere system changes significantly on various temporal scales due to the forcings from solar and geomagnetic processes, and the lower atmosphere. The 27-day variation caused by solar rotation is one of the most important modulating factors in the ionosphere. A robust feature in this context is the ionospheric lag of about 1-2 days in ionospheric parameters such as total electron content (TEC) and F2 layer peak electron density with respect to solar variations at the 27-day solar rotation period. Here, the ionospheric TEC provided by the International GNSS Service (IGS) and the Coupled Thermosphere Ionosphere Plasmasphere electrodynamics (CTIPe) model were used to understand the delayed ionospheric response and the underlying physics. The O/N₂ measurements from the imaging ultraviolet spectrograph Global-Scale Observations of the Limb and Disk (GOLD), the Global Ultraviolet Imager (GUVI), were also analyzed during the 2019-2021 period of low solar activity. The comparative study shows that the model successfully reproduces the delayed response of the ionosphere during low solar activity. The observed and modeled O/N₂ ratio was found to be positively correlated with the solar EUV proxy (GOLD Q_EUV), with a lag of about 2 days, indicating a contribution to the ionospheric lag in TEC.
Furthermore, the CTIPe model simulations show that the ionospheric time delay is significantly affected by various physical processes such as diffusion, photodissociation, solar and geomagnetic activities, and wave dynamics.